Thermal copying means employing open-celled microporous film

ABSTRACT

Thermally sensitive master sheet for use in multiple reproduction processes having open-celled microporous films with parallel fibrils. When exposed during use the heat generated by image areas of an original produces coalescence of the microporous voids within the film thereby clarifying the film at that site with a high definition transfer of the original. With cold stretching with or without subsequent hot stretching, with heat set, possess good resistance to heat, moisture and normal pressure of handling.

United States Patent 1191 Plovan Oct. 9, 1973 [54] THERMAL COPYING MEANS EMPLOYING 3,298,895 1/1967 Plambeck 101/327 X O EN E MICRQPOROUS FILM 2,440,102 4/1948 Land 101/426 X I 2,371,868 3 1945 Berg etal... l0l/367X Inventor: Steven a Llvmgston, 3,547,753 12/1970 Sutton 161/406 x [73] Assignee: Celanese Corporation, New Yrok,

NY. Primary Examiner-Clyde I. Coughenour Filed: p 1971 .fltltzrzggl'gggrglas J. Morgan, Charles B. Barns and [21] Appl. No.: 134,428

[57] ABSTRACT [52] Cl i 3 4 Thermally sensitive master sheet for use in multiple [51] I t Cl 1 1 6 5 13/00 reproduction processes having open-celled micropo- [58] i l(')"1/47l473 128 4 rous films with parallel fibrils. When exposed during e 0 care l17/355 3 use the heat generated by image areas of an original produces coalescence of the microporous voids within the film thereby clarifying the film at that site with a [56] References cued high definition transfer of the original. With cold UNITED STATES PATENTS stretching with or without subsequent hot stretching, 3,530,792 9/1970 Valiela 101/115 X with heat set, possess good resistance to heat, mois- 3,022,542 2/1962 Davis l0l/327 X ture and normal pressure of handling. 3,426,754 2/1969 Bierenbaum et al.. 128/156 3,055,297 9/ 1962 Leeds 101/327 8 Claims, No Drawings THERMAL COPYING MEANS EMPLOYING OPEN-CELLED MICROPOROUS FILM BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to novel materials for the thermally responsive reception, recording, and storage of image information and relates to improved heat sensitive materials from which negative copies of imaged original sheets can be produced by heat energy modification of the'internal void structure of certain open-celled microporous films.

2. Description of the Prior Art Although it is highly desirable to utilize a single polymeric thermo-sensitive film in lieu of the use of a thermo-sensitive coating upon a stable substrate in many recording and data duplication applications because of the inherent advantages of strength, handleability, moisture resistance, and stability, certain problems arise when attempting this desired substitution. Two inherent disadvantages reside in the fact that 1. it has not been economically feasible to produce a polymeric film useful as a master for duplication purposes entirely out of a thermally sensitive material; and

2. thermal-polymeric masters usually lack sufficient sensitivity, i.e., they require either high temperatures or relatively long exposure times at a specific temperature.

An approach to obviating these reproduction difficulties has been made by the production of recording papers with thermo-sensitive, opaque coatings of fully enclosed microscopic voids which when locally heated, as for example with a hot stylus, soften and coalesce with attendant collapse of the voids and release of air therefrom to form a homogeneous, relatively transparent mass disclosing the underlying surface at the area affected by the stylus as illustrated in U.S. Pat. No. 2,739,909.

Another cavernous, pressure-clarifiable coating has also been produced by applying an emulsion to a substrate in the form of a continuous liquid phase in which is dispersed another liquid forming a discontinuous phase. The air-matrix interfaces of the enclosed pores in this film scatter incident light resulting in an opaque coating which, when subject to sharp pressure in the presence of a transparentizing" agent, realizes an extremely thin translucent or transparent covering. This type of coating is fully described in U.S. Pat. No. 2,961,334.

Also, thin opaque coatings of polymeric foam have been produced as in U.S. Pat. No. 3,145,117, which, under localized pressure, become more or less translucent, providing a conspicuously contrasting mark when attached to a dark underlying paper.

Additional attempts to obtain porous films have heretofore involved combination of melt extrusion or solution processes with blowing agents or foaming techniques, which are difficult to control to give reproduceable porosity. Also, porosity obtained by such methods is generally of the enclosed void type, the films of which do not have a high degree of permeability or receptivity for aqueous solutions; nor does pressure cause clarification.

Porous, open-celled films of addition polymers which are opaque yet pressure-clarifiable have been prepared as in U.S. Pat. No. 2,957,791 by partially coalesing discrete polymer particles from either an aqueous-solvent dispersion or an aqueous metal salt media. Open-celled films, such as these, usually possess a high degree of receptivity to both aqueous and non-aqueous dyes, irrespective of the specific polymer from which they are made.

It would be highly desirable to have a thermally sensitive polymeric film which could be rapidly imaged by exposure to light radiation alone yet possess good stability under normal storage conditions; stability in normal ultraviolet radiation, e.g., sunlight; permanence under normal physical handling, i.e., will not smudge; and possess general immunity from the chemical actions of aqueous or common solvent solutions. In addition, it would also be desirable to keep the film production process simple by avoiding, for example, coagulation steps; to keep the reproduction process simple by eliminating the need for chemical reaction; and to keep the final product inexpensive by eliminating the need for expensive thermally sensitive chemicals such as ther rnochromic organic materials.

While the aforementioned cellular film coatings of the prior art are useful, the search has continued for new processes able to produce thermally sensitive, open-cell miroporous films having a greater number of pores; a more uniform pore concentration or distribution; a larger total pore area; and good ambient stability of the void volume.

By way of introduction to the concept of the instant invention it is understood that most, if not all, materials in nature are infrared absorptive, but of these materials not all are infrared absorptive to the same degree. Due to the absorption of infrared radiations and the consequent conversion of the radiation into heat energy, the body or material experiences a rise in temperature dependent on the substance, intensity and time of exposure. Where infrared radiation absorbing images are carried on a sheet having lesser infrared absorbing materials, the controlled exposure of this sheet to infrared radiations will cause the generation of a differential heat pattern in the sheet, with the areas in the sheet corresponding to the infrared radiation absorbing images being elevated to the highest heat level or temperature and the other aras being elevated to a lesser heat level or temperature.

For example, this sheet of paper is white and has impressed thereupon dark images in the form of typed characters so that it defines an original sheet having imaged infrared absorptive areas and non-imaged lesser infrared absorptive areas. In the non-imaged areas lesser amounts of any incident infrared radiations are absorbed, while the greater portion of any incident infrared radiations is either reflected from the surface or transmitted through the sheet. Within the infrared absorbing imaged areas, the converse is true, that is, a greater portion of any incident infrared radiation is absorged therein and a lesser portion of any incident infrared radiation is either reflected or transmitted on therethrough. Upon irradiation from a controlled infrared source for an exact period and due to the ensuing absorption, the sheet suffers an overall temperature elevation with a lesser and substantially uniform temperature elevation taking place within the non-imaged areas and with a greater and substantially delineated temperature elevation taking place within the imaged areas.

Thus, there is developed in the original sheet a dominant heat pattern corresponding identically to the distinctive infrared radiation absorption pattern of the imaged original sheet. This dominant heat pattern can be utilized for purposes of selectively activating a temperature sensitive polymer material. As an aside, it should be noted that in any practical thermo-transfer process, it is necessary that the heat pattern developed in the original sheet be transferred immediately, and with an exactitude corresponding to the original, to the thermally sensitive material. Specifically, it is necessary that the original be in close heat conductive relationship with the material throughout the interval required to infrared radiate the original; generate the differential heat pattern therein; and conduct the generated heat pattern undisturbed to the transfer material. But, for this process to be effective and efficient, all of these actions must take place substantially simultaneously, under a condition of intimate contact.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a new and improved polymeric film which is adapted for making negative single copies or masters from a graphic original in a thermo-transfer process.

It is another object of this invention to provide a heat-clarifiable polymer film which, under the influence of a heat pattern, is rendered transparent in only the critically heated areas so that a sharp, discrete transparent character corresponding to the image area is produced leaving the uncritically heated adjacent areas undisturbed, i.e., opaque or translucent.

It is another object of the invention to provide a thermally sensitive polymeric film which can produce a single negative copy of an original in a matter of seconds leaving the original unchanged.

A more specific object of the invention is to provide a thermally sensitive polymeric film which can be interposed between a strong radiation source rich in infrared energy and an imaged original of which a copy is to be made, in a manner so as to transfer sufficient quantities of the incident radiation so that only the areas of the polymeric film corresponding to the imaged areas on the original become transparent and the other areas corresponding to the non-imaged areas of the original remain opaque or translucent.

Other and further objects of the present invention will be apparent to those skilled in the art from the following:

DETAILED DESCRIPTION OF THE INVENTION Porous or cellular films can be classified into two general types: one type in which the pores are not interconnected, i.e., a closed-cell film, and the other type in which the pores are essentially interconnected through tortuous paths which may extend from one exterior surface or region to another, i.e., an open-celled film. The porous films of the present invention are of the latter type.

The microporous films useful in the present invention are also characterized by a reduced bulk density, sometimes hereinafter referred to simply as a low density. That is, these microporous films have a bulk or overall density lower than the bulk density of corresponding films composed of identical polymeric material but having no open-celled or other voidy structure. The term bulk density" as used herein means the weight per unit of gross or geometric volume of the film, where gross volume is determined by immersing a known weight of the film in a vessel partly filled with mercury at 25 C. and atmospheric pressure. The volumetric rise in the level of mercury is a direct measure of the gross volume. This method is known as the mercry volumenometer method, and is described in the Encyclopedia of Chemical Technology, Vol. 4, page 892 (Interscience 1949).

Porous films have been produced which possess a microporous, open-celled structure, and which are also characterized by a reduced bulk density. Films possessing this microporous structure are described, for example, in U.S. Pat. No. 3,426,754 which patent is assigned to the assignee of the present invention. The preferred method of preparation described therein involves drawing or stretching at ambient temperatures, i.e., cold drawing, a crystalline, elastic starting film in an amount of about 10 to 300 percent of its original length, with subsequent stabilization by heat setting of the drawn film under a tension such that the film in not free to shrink or can shrink only to a limited extent.

While the above described microporous or voidcontaining film of the prior art is useful in this invention the search has continued for new processes able to produce open-celled microporous films having a greater number of pores, a more uniform pore concentration or distribution, a larger total pore area, and better thermal stability of the porous or voidy film. These properties are significant in applications such as filter media where a large number of uniformly distributed pores are necessary or highly desirable; and in applications such as breathable medical dressings subject to high temperatures, e.g., sterilization temperatures, where thermal stability is necessary or highly desirable.

An improved process for preparing the transparentizable open-celled microporous polymer films from nonporous, crystalline, elastic polymer starting films, includes 1) cold stretching, i.e., cold drawing the elastic film until porous surface regions or areas which are elongated normal or perpendicular to the stretch direction are formed, (2) hot stretching, i.e., hot drawing, the cold stretched film until fibrils and pores or open cells which are elongated parallel to the stretch direction are formed, and thereafter (3) heating or heat setting the resulting porous film under tension, i.e., at substantially constant length, to impart stability to the film. Yet another process is similar to this process but consolidates steps (2) and (3) into a continuous simultaneous, hot stretching-heat setting step, said step being carried out for a time sufficient to render the resulting microporous film substantially (less than about 15 percent) shrink resistant.

The elastic starting film or precursor film is preferably prepared from crystalline polymers such as polypropylene by melt extruding the polymer into a film, taking up the extrudate at a drawdown ratio giving an oriented film, and thereafter heating or annealing the oriented film if necessary to improve or enhance the initial crystallinity.

The essence of the improved processes is the discovery that the sequential cold stretching and hot stretching steps impart to the elastic film a unique open-celled structure which results in advantageous properties, including improved porosity, improved thermal stability and a gain or enhancement of porosity when treated with certain organic liquids such as perchloroethylene.

As determined by various morphological techniques or tests such as electron microscopy, the microporous filmsof the improved process are characterized by a plurality of elongated, non-porous, interconnecting surface regions or areas which have their axes of elongation substantially parallel. Substantially alternating with and defined by these non-porous surface regions are a plurality of elongated, porous surface regions which contain a plurality of parallel filbrils or fibrous threads. These fibrils are connected at each of their ends to the non-porous regions, and are substantially perpendicular to them. Between the fibrils are the pores or open cells of the films utilized by the present invention. These surface pores or open cells are substantially interconnected through tortuous paths or passageways which extend from one surface region to another surface area or region.

With such a defined or organized morphological structure, the films which are treated according to the instant process may have a greater proportion of surface area that the pores cover, a greater number of pores, and a more uniform distribution of pores, than previous microporous films. Further, the fibrils present in the films of the present invention are more drawn or oriented with respect to the rest of the polymer material in the film, and then contribute to the higher thermal stability of the film.

In fact the total surface area per cubic centimeter of material of the films of this invention have a range of from 2 to about 200 square meters per cc. Preferably the range is from about 5 to about 100 square meters per cc. and more preferably from about to about 80 square meters per cc. These values can be compared with normal pin-holed film which has a total surface area per gram of about 0.1 square meters; paper and fabric which have values per gram of about 1.0 square meters and leather which has a value of about 1.6 square meters per cc. Additionally, the volume of space per volume of material ranges from about 0.05 to about 1.5 cubic centimeters per gram, preferably from about 0.1 to about 1.0 cubic centimeters per gram and most preferably from 0.2 to about 0.85 cubic centimeters per gram. Additionaldata to define the films of this invention relates to nitrogen flux measurements, wherein the microporous films have 0 (or nitrogen) Flux values in the range of from about 5 to 400 preferably about 50 to 300. These values give an indication of porosity, with higher nitrogen flux values indicating higher levels of porosity.

Nitrogen flux may be calculated by mounting a film having a standard surface area of 6.5 square centimeters in a standard membrane cell having a standard volume of 63 cubic centimeters. The cell is pressurized to a standard differential pressure (the pressure drop across the film) of 200 pounds per square inch with nitrogen. The supply of nitrogen is then closed off and the time required for the pressure to drop to a final differential pressure of 150 pounds per square inch as the nitrogen permeates through the film is measured with a stop watch. The nitrogen flux, Q, in gram moles per square centimeter minute, is then determined from the equation:

0 27.74 X ID /At X T where At is the change in time measured in seconds and T is the temperature of nitrogen in degrees Kelvin. The

above equation is derived from the gas law, PV Z,,RT.

The microporous films used in the present invention are formed from a starting elastic film of crystalline, film-forming, polymers. These elastic films have an elastic recovery at zero recovery time (hereinafter defined) when subjected to a standard strain (extension) of 50 percent at 25 C. and 65 percent relative humidity of at least about 40 percent, preferably at least about 50 percent, and most preferably at least about percent.

Elastic recovery as used herein is a measure of the ability of a structure or shaped article such as a film to return to its original size after being stretched, and may be calculated as follows:

Elastic Recovery(ER)=[(length when stretched length after stretching X 100/Length added when when stretched] Although a standard strain of 50 percent is used to identify the elastic properties of the starting films, such strain is merely exemplary. In general, such starting films will have elastic recoveries higher at strains less than 50 percent, and somewhat lower at strains substantially higher than 50 percent, as compared to their elastic recovery at a 50 percent strain.

These starting elastic films will also have a percent crystallinity of at least 20 percent, preferably at least 30 percent and most preferably at least 50 percent, e.g., about 50 to percent, or more. Percent crystallinity is determined by the x-ray method described by R. G. Quynn et al. in the Journal of Applied Polymer Science, Vol. 2 No. 5 pp 166-173 (1959). For a detailed discussion of crystallinity and its significance in polymers, see Polymers and Resins, Golding (D. Van Nostrand, 1959).

Preferred suitable starting elastic films, as well as the preparation thereof, are further defined in British Pat. No. 1,198,695, published July 15, 1970. Other elastic films which may be suitable for the practice of the present invention are described in British Pat. No. 1,052,500, published Dec. 21, 1966 and are well known in the art.

The starting elastic films utilized in the preparation of the microporous films of the present invention should be differentiated from films formed from classical elastomers such as the natural and synthetic rubbers. With such classical elastomers the stress-strain behavior, and particularly the stress-temperature relationship, is governed by entropy-mechanism of deformation (rubber elasticity). The positive temperature coefficient of the retractive force, i.e., decreasing stress with decreasing temperature and complete loss of elastic properties at the glass transition temperatures, are particularly consequences of entropy-elasticity. The elasticity of the starting elastic films utilized herein, on the other hand, is of a different nature. In qualitative thermodynamic experiments with these elastic starting films, increasing stress with decreasing temperature (negative temperature coefficient) may be interpreted to mean that the elasticity of these materials is not governed by entropy effects but dependent upon an energy term. More significantly, the starting elastic films have been found to retain their stretch properties at temperatures where normal entropy-elasticity could no longer be operative. Thus, the stretch mechanism of the starting elastic films is thought to be based on energyelasticity relationships, and these elastic films may then be referred to as non-classical elastomers.

As stated, the starting elastic films employed in this invention are made from a polymer of a type capable of developing a significant degree of crystallinity, as contrasted with more conventional classical elastic materials such as the natural and synthetic rubbers which are substantially amorphous in their unstretched or tensionless state.

A significant group of polymers, i.e., synthetic resinous materials, to which this invention may be applied are the olefin polymers, i.e., polyethylene, polypropylene, poly-3-methyl butene-l, poly-4-methyl pentene-l, as well as copolymers of propylene, 3-methyl butene-l, 4-methyl pentene-l, or ethylene with each other or with minor amounts of other olefins, e.g., copolymers of propylene and ethylene, copolymers of a major amount of 3-methyl butene-l and a minor amount of a straight chain n-alkene such as n-octene-l, n-hexadecene-l, n-octadecene-l or other relatively long chain alkenes, as well as copolymers of 3-methyl pentene-l and any of the same n-alkenes mentioned previously in connection with 3-methyl butene-l. These polymers in the form of film should generally have a percent crystallinity of at least 20 percent, preferably at least 30 percent, and most preferably about 50 percent to 90 percent or higher.

For example, a film-forming homopolymer of polypropylene may be employed. When propylene homopolymers are contemplated, it is preferred to employ an isotactic polypropylene having a percent crystallinity as indicated above, a weight average molecular weight ranging from about 100,000 to 750,000 preferably about 200,000 to 500,000 and a melt index (AST- Ml,958D-l,23857T, Part 9, page 38) from about 0.1 to about 75, preferably about 0.5 to 30, so as to give a final film product having the requisite physical properties.

While the present disclosure and examples are directed primarily to the aforesaid olefin polymers, the invention also contemplates the high molecular weight acetal, e.g., oxymethylene; polymers. While both acetal homopolymers and copolymers are contemplated, the

preferred acetal polymer is a random" oxymethylene copolymer, one which contains recurring oxymethylene, i.e., Cl-l -O-, units interspersed with -OR groups in the main polymer chain where R is a divalent radical containing at least two carbon atoms directly linked to each other and positioned in the chain between the two valences, with any substituents on said R radical being inert, that is, those which do not include interfering functional groups and which will not induce undesirable reactions, and wherein a major amount of the OR units exist as single units attached to oxymethylene groups on each side. Examples of preferred polymers include copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms such as the copolymers disclosed in U.S. Pat. No. 3,027,352 of Walling et al. These polymers in film form may also have a crystallinity of at least 20 percent, preferably at least 30 percent, and most preferably at least percent, e.g., 50 to percent or higher. Further, these polymers have a melting point of at least C., and a number average molecular weight of at least 10,000. For a more detailed discussion of acetal and oxymethylene polymers, see Formaldehyde, Walker, pp. -191, (Reinhold 1964).

Other relatively crystalline polymers to which the invention may be applied are the polyalkylene sulfides such as polymethylene sulfide and polyethylene sulfide, the polyarylene oxides such as polyphenylene oxide, the polyamides such as polyhexamethylene adipamide (nylon 66) and polycaprolactam (nylon 6), and polyesters such as polyethylene terephthalate, all of which are well known in the art and need not be described further herein for sake of brevity.

The types of apparatus suitable for forming the starting elastic films of this invention are well known in the art.

For example, a conventional film extruder equipped with a shallow channel metering screw and coat hanger die, is satisfactory. Generally, the resin is introduced into a hopper of the extruder which contains a screw and a jacket fitted with heating elements. The resin is melted and transferred by the screw to the die from which it is extruded through a slot in the form of a film from which it is drawn by a take-up or casting roll. More than one take-up roll in various combinations or stages may be used. The die opening or slot width may be in the range, for example, of about 10 to 200 mils.

Using this type of apparatus, film may be extruded at a drawndown ratio of about 20:1 to 200:1, preferably 50:1 to 150:1.

The terms drawdown ratio or, more simply, draw ratio, as used herein is the ratio of the film wind-up or take-up speed to the speed of the film issuing at the extrusion die.

The melt temperature for film extrusion, is in general, no higher than about 100 C. above the melting point of the polymer and no lower than about 10 C. above the melting point of the polymer.

For example, polypropylene may be extruded at a melt temperature of about C. to 270 C., preferably 200 C. to 240 C. Polyethylene may be extruded at a melt temperature ofabout 175 C. to 225 C., while acetal polymers, e.g., those of the type disclosed in U.S. Pat. No. 3,027,352 may be extruded at a melt temperature of about C. to 235 C., preferably C. to 215 C.

The extrusion operation is preferably carried out with rapid cooling and rapid drawdown in order to obtain maximum elasticity. This may be accomplished by having the take-up roll relatively close to the extrusion slot, e.g., within two inches and, preferably, within one inch. An air knife operating at temperatures between, for example 0 C. and 40 C., may be employed within one inch of the slot to quench, i.e., quickly cool and solidify the film. The take-up roll may be rotated, for example, at a speed of 10 to 100 ft/min. preferably to to 500 ft/min.

While the above description has been directed to slit die extrusion methods, an alternative method of forming the starting elastic films contemplated by this invention is the blown film extrusion method wherein a hopper and an extruder are employed which are substantially the same as in the slot extruder described above. From the extruder, the melt enters a die from which it is extruded through a circular slot to form a tubular film having an initial diameter D Air enters the system through an inlet into the interior of said tubular film and has the effect of blowing up the diameter of the tubular film to a diameter D Means such as air rings may also be provided for directing the air about the exterior of extruded tubular film so as to provide quick and effective cooling. Means such as cooling mandrel may be used to cool the interior of the tubular film. After a short distance during which the film is allowed to completely cool and harden, it is wound up on a take-up roll.

Using the blown film method, the drawdown ratio is preferably 20:1 to 200:1, the slot opening 10 to 200 mils, the D /D ratio, for example, 0.5 to 6.0 and preferably about 1.0 to about 2.5, and the take-up speed, for example, 30 to 700 ft/min. The melt temperature may be within the ranges given previously for straight slot extrusion.

The extruded film may then be initially heat treated or annelaed in order to improve crystal structure, e.g., by increasing the size of the crystallites and removing imperfections therein.

In order to render the precursor or starting film microporous, it is subject to a process generally comprising the steps of stretching and heat setting the starting film. Preferably the process comprises either the consecutive steps of cold stretching, hot stretching and heat setting or the steps of cold stretching and simultaneously hot stretching and heat setting the precursor film. Other variations on this process (such as elimination of the hot stretching step) can be carried out resulting in microporous films which, although slightly inferior to those films made by the cold stretch hot stretch heat set process, still find utility as the microporous films of this invention.

The term cold stretching" as used herein is defined as stretching or drawing a film to greater than its original length and at a stretching temperature, i.e., the temperature of the film being stretched, less than the temperature at which the melting of the film begins when the film is uniformly heated from a temperature of 25 C. at a rate of 20 C. per minute. The terms hot stretching or hot stretching-heat setting as used herein is defined as stretching above the temperature at which melting begins when the film is heated from a temperature of 25 C. at a rate of 20 C. per minute, but below the normal melting point of the polymer, i.e., below the temperature at which fusion occurs. For example, using polypropylene elastic film, cold stretching is carried out preferably below about 120 C. while hot stretching or hot stretching-heat setting is carried out above is temperature.

When a separate heat setting step is employed it follows the cold stretching heat stretching steps and is carried out at from about 125 C. up to less than the fusion temperature of the film in question. For polypropylene the range preferably is about 130 C. to 160 C.

The total amount of stretching or drawing which should occur when either a single stretching or consecutive stretching steps occur is in the range of about 10 to about 300 percent of the original length of the film prior to stretching.

The resulting microporous film exhibits a final cyrstallinity of preferably at least 30 percent, more preferably about 50 to 100 percent as determined by the aforementioned x-ray method and as previously defined an elastic recovery from a 50 percent strain of at least 50 percent preferably 60 to 85 percent. Furthermore, this film exhibitsan average pore size of about 100 to 12,000 angstroms more usually 150 to 5,000 angstroms, the values being determined by mercury porosimetry as described in an article by R.G. Quynn et al., on pages 21-34 of Textile Research Journal, January 1963.

The opaque, open-celled microporous film is then placed in an intermediate position between an infrared radiation source and an imaged original sheet such that the radiation impinges on the forward face of the microporous film and the original is in intimately close association with the rear face. The infrared radiation penetrates the microporous film being selectively absorbed by the infrared absorbing materials: particularly by the imaged areas of the original sheet.

The film will develop a first heat due to infrared absorption of the material in the non-image areas of the original and the polymeric material of the film itself. To this heat is added the image heat due to the conductive heat image generated on the original sheet. Thus, on the surface of the microporous film contiguous to the face of the original, there is developed a heat pattern which consists of the first heat as described above, and a second or accumulative heat made up of the first heat plus the image heat and corresponds to the image areas of the original. With proper infrared radiation control, the first heat can be limited to generate at the contiguous copying surface a temperature which is below the critical temperature at which the microporous cells coalesce whereas the second accumulative heat at that surface will be sufficient to generate a temperature at or about this critical fusion temperature. This results in a selective coalescence of the microporous voids in the film to a discrete depth at the surface contiguous to the copying surface and of a breadth including only those discrete areas corresponding to the imaged areas of the original. This coalesced area is now void free, i.e., transparent, as opposed to the unaffected opaque microporous areas of the film.

It is to be noted that the amount of heat generated depends upon the infrared absorption characteristics of the material in the film and the original, and the amount of infrared radiation as a function of time and intensity to which the combinationis subjected. lt is appreciated that if the open-cell microporous film is subjected to too great an exposure of infrared radiation, or if it is comprised of material that is highly absorptive of infrared radiation, the heat developed therein may conflict with, override, or obliterate the temperature pattern conducted thereto from the original. The microporous material must be able to differentiate between the temperature elevations generated therein and the heat image conducted thereto from the original sheet.

The heat image developed in the original sheet must be transmitted substantially undistorted and undiminished back to heat at least the surface of the microporous film contiguous to the copying surface to a temperature sufficient to soften and coalesce the voids in the heat pattern areas. This means that the conduction of the heat image must be without any substantially conflicting heat patterns being developed. This is critical for the purpose of maintaining good copy definition, it being understood, that with lateral diffusion of the heat there is not only an attenuation or diminution of the peak temperature from that developed from the heat image of the original but that there is also a tendency to spread the heat image, thereby developing a dispersed heat image of substantially greater area than the heat image originally generated in the original sheet and causing fill-in of the image characters.

The excellent definition of the characters produced by the open-celled microporous films of the instant invention is a direct result of the fine uniformity and extrenely small pores in this film; the average pore size usually being less than 5,000 Angstroms. To analogize, this occurrence is not unlike the enhanced sharpness of detail realizable by the use of increasingly finer grain in photographic film.

EXAMPLE I Crystalline polypropylene having a melt index of 0.7 and a density of 0.92 is melt extruded at 230 C.

. through an 8 inch slit die of the coat hanger type using a 1 inch extruder with a shallow metering screw. The length to diameter ratio of the extruder screw is 24/1. The extrudate is drawndown very rapidly to a melt drawdown ratio of 150, and contacted with a rotating casting roll maintained at 50 C. and 0.75 inches from the lip of the die. The film produced in this fashion is found to have the following properties: thickness, 0.001 inches; recovery from 50 percent elongation at 25 C., 50.3 percent; cry'stallinity, 59.6 percent.

A sample of this film is oven annealed in air with a slight tension at 140 C. for about 30 minutes, removed from the oven and allowed to cool. It is then found to have the following properties: recovery from a 50 percent elongation at 25 C., 90.5 percent; crystallinity, 68.8 percent.

The annealed film is first cold stretched at 25 C. and thereafter hot stretched at 145 C. Total stretch is 100 percent, based on the original length of the film, and the extension ratio is 0.90. Nitrogen flux (at 65 C.) of the resulting open-celled microporous film is 127.5 X gram moles per cm 2 min.

An original sheet bearing transferable images is placed in the base of an exposure device of the vacuum frame type with the imaged surface facing an infrared radiation source. This heat energy source consists of a 250 Watt infrared lamp plus reflector and possesses a controllable tranversal rate. The distance from the imaged surface to the infrared source is adjustable from approximately 0.5 to 12 inches. When a lamp voltage of 77 volts is utilized the preferred distance is from about 0.5 to 3 inches and when l 10 volts is utilized, the preferred distance is from about 3 to about 12 inches. A blower is also employed within the apparatus for maintaining it at substantially ambient temperature.

The above-prepared opaque open-celled microporous film with a nominal thickness of 0.001 inches is placed upon the imaged original and this laminate compressed to about 5 pounds per sq. in. by a Grade 3 glass plate.

For the production of a master sheet carrying thereon a sharply defined transparent image of full breadth and depth upon an unaffected opaque field, the exposure rate was found to be 1 to 12 and preferably about 2 to 3 inches per second at 100 volts and a source to impact range of approximately 3 to 6 inches. Severe over-exposure results in actual melting within the image areas.

The characteristics of the original are not important other than it be capable of developing a dominant heat image corresponding to the image thereon, and that the sheet itself be substantially non-absorbent of radiation.

Specifically, exposure in accordance with the teaching of the invention results in selective coalescence of the microporous material at the surface contiguous to the original of a breadth corresponding only to the imaged areas of the original resulting in excellent border definition.

EXAMPLE II The resulting negative of Example I is used for the production of sharp, positive photo-prints on photographic paper and for the production of photographic transparencies. It also serves as an excellent lantern slide for projection of bright line characters (caused by the blocking of the projected light by the non-clarified opaque areas of the microporous film).

EXAMPLE III The film-forming polymer of this example is a copolymer of trioxane and 2 weight percent, based on the weight of the polymer, of ethyelne oxide of the type described in US. Pat. No. 3,027,352, which is aftertreated to remove unstable groups as described in US. Pat. No. 3,219,623, and which has a melt index of 2.5.

The above-described polymer is melt extruded at 195 C. through an 8 inch slit of the coat hanger type using a 1 inch extruder with a shallow channel metering screw. The length to diameter ratio of the extruder screw was 24:1. The extrudate is drawn down to a drawdown ratio of 150:1, contacted with a rotating casting roll maintained at about 145 C., and about one-quarter inch from the lip of the die. The film produced in this manner is wound up and found to have the following properties: thickness, 0.0005 inches; recovery from 50 percent strain, 45 percent. The polymer is then oven annealed in the tensionless state at 145 C. for 16 hours. At the end of the annealing period it is removed from the oven, allowed to cool and found to have the following properties: thickness, 0.0005 inches; recovery from 50 percent elongation, 92 percent.

The film is cold stretched at 25 C. to 10 percent of its original length, and thereafter stretched at C. to a total extension of 100 percent of its original length, and thereafter heat set at constant length in an oven at C. for 2 minutes. At the end of this period it is removed from the oven, allowed to cool and found to have the open-celled microporous structure of the present invention.

This microporous film is placed between the imaged surface of an original sheet and the glass plate of an exposure device as described in Example 1.

By operating the exposure unit to provide a traversal at the rate of l to 12 and preferably 2 to 3 inches per second at l 10 volts and a source to impact range of approximately 3 to 6 inches, there is produced a master sheet carrying thereon a sharply delineated transparent image.

The negative thus prepared is used to expose a photopolymer printing plate. Printings from this plate are of high quality and definition.

EXAMPLE IV The open-cell microporous film and the exposure device of Example I is again employed; however, in lieu of the positively imaged original a negative of the same is utilized. By operating the exposure device to provide a traversal at the rate of approximately 1 to 12 and preferably 2 to 3 inches per second, as in the aforementioned example, there is produced a sharply delineated positive" master sheet carrying thereon a transferred positive image of full breadth and depth.

EXAMPLE V The master polymeric film product of Example IV consists of transparent non-porous non-imaged areas and opaque open-celled microporous imaged areas. The inking of these master sheets with dyes of high tinctorial strengths, such as methyl violet, crystal violet, etc., and their subsequent use in a multiplereproduction unit results in printings of outstanding quality and reproduceability.

Fillers such as dyes, flame inhibitors, heat sensitive agents, etc., may also be added to change the properties of the final microporous film. However, these are merely incidental and large amounts of such fillers are generally not preferred.

In conclusion, what is disclosed is a thermal copying means utilizing an inherently inexpensive and easily produced polymer material which, because of its low mass and extremely fine pore size and uniformity, is of sufficient thermal sensitivity to rapidly produce images of excellent definition.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the present invention.

What is claimed is:

1. An unbacked master copy for use as a spirit type duplicating sheet which comprises an open-celled microporous polymer film characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above about 20 percent, a pore size of mo to 12,000 Angstroms, a nitrogen flux of 400, and an elastic recovery at 50 percent extension of greater than 40 percent; bearing thereon nonporous, transparent images, said film being further characterized by having a plurality of elongated nonporous, interconnecting surface regions which have their axis of elongation substantially parallel; substantially alternating with and defined by these non-porous surface regions a plurality of elongated porous surface regions which contain a plurality of parallel fibrils which are connected at each of their ends to the nonporous regions and are substantially perpendicular to them; and between the fibrils pores or open cells which are substantially interconnected through torturous paths which extend from one surface region to another surface region.

2. A master copy for use as a spirit type duplicating sheet according to claim 1 wherein the open-celled microporous film comprises a film of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, a bulk density of 50 to 75 percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, and a surface area of at least 30 sq. m/cc.

3. An unbacked polymeric copy which consists essentially of an open-celled microporous polymer film characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above about 20 percent, a pore size of 10.0 to 12,000 Angstroms, a nitrogen flux of 5-400 and an elastic recovery at 50 percent extension of greater than 40 percent; bearing thereon non-porous, transparent images; said film being further characterized by having a plurality of elongated non-porous, interconnecting surface regions which have their axis of elongation substantially parallel; substantially alternating with and defined by these non-porous surface regions a' plurality of elongated porous surface regions which contain a plurality of parallel fibrils which are connected at each of their ends to the non-porous regions and are substantially perpendicular to them; and between the fibrils pores or open cells which are substantially interconnected through torturous paths which extend from one surface region to another surface region.

4. A polymeric copy according to claim 3 wherein the open-celled microporous film comprises a film of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, a bulk density of 50 to percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, and a surface area of at least 30 sq. m/cc.

5. An unbacked master copy for use as a spirit type duplicating sheet which consists essentially of opencelled microporous polymeric film images characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above 20 percent, a pore size of to 12,000 Angstroms, a nitrogen flux of 5-400 and an elastic recovery at 50 percent extension of greater than 40 percent; upon a non-porous, transparent background of the same polymeric material, said film being further defined by having a plurality of elongated non-porous, interconnecting surface regions which have their axis of elongated substantially parallel; substantially alternating with and defined by these non-porous surface regions a plurality of elongated porous surface regions which contain a plurality of parallel fibrils which are connected at each of their ends to the non-porous regions and are substantially perpendicular to them; and between the fibrils pores or open cells which are substantially interconnected through torturous paths which extend from one surface region to another surface region.

6. A master copy for use as a spirit type duplicating sheet according to claim 5 wherein the open-celled microporous film comprises a film of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyester, polyalkylene sulfides and polyarylene oxides, a bulk density of 50 to 75 percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, and a surface area of at least 30 sq. m/cc.

7. An unbacked polymeric copy which consists essentially of open-celled microporous polymeric film images characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above 20 percent, a pore size of 100 to 12,000 Angstroms, a nitrogen flux of about 5-400 and an elastic recovery at 50 percent extension of greater than 40 percent; upon a non-porous, transparent background of the same polymeric material, said film being further defined by having a plurality of elongated non- 8. A polymeric copy according to claim 7 wherein the open-celled microporous film image comprises a film of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, a bulk density of 50 to percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, and a surface area of at least 30 sq. m/cc. 

2. A master copy for use as a spirit type duplicating sheet according to claim 1 wherein the open-celled microporous film comprises a film of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, a bulk density of 50 to 75 percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, and a surface area of at least 30 sq. m/cc.
 3. An unbacked polymeric copy which consists essentially of an open-celled microporous polymer film characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above about 20 percent, a pore size of 100 to 12,000 Angstroms, a nitrogen flux of 5-400 and an elastic recovery at 50 percent extension of greater than 40 percent; bearing thereon non-porous, transparent images; said film being further characterized by having a plurality of elongated non-porous, interconnecting surface regions which have their axis of elongation substantially parallel; substantially alternating with and defined by these non-porous surface regions a plurality of elongated porous surface regions which contain a plurality of parallel fibrils which are connected at each of their eNds to the non-porous regions and are substantially perpendicular to them; and between the fibrils pores or open cells which are substantially interconnected through torturous paths which extend from one surface region to another surface region.
 4. A polymeric copy according to claim 3 wherein the open-celled microporous film comprises a film of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, a bulk density of 50 to 75 percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, and a surface area of at least 30 sq. m/cc.
 5. An unbacked master copy for use as a spirit type duplicating sheet which consists essentially of open-celled microporous polymeric film images characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above 20 percent, a pore size of 100 to 12,000 Angstroms, a nitrogen flux of 5-400 and an elastic recovery at 50 percent extension of greater than 40 percent; upon a non-porous, transparent background of the same polymeric material, said film being further defined by having a plurality of elongated non-porous, interconnecting surface regions which have their axis of elongation substantially parallel; substantially alternating with and defined by these non-porous surface regions a plurality of elongated porous surface regions which contain a plurality of parallel fibrils which are connected at each of their ends to the non-porous regions and are substantially perpendicular to them; and between the fibrils pores or open cells which are substantially interconnected through torturous paths which extend from one surface region to another surface region.
 6. A master copy for use as a spirit type duplicating sheet according to claim 5 wherein the open-celled microporous film comprises a film of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyester, polyalkylene sulfides and polyarylene oxides, a bulk density of 50 to 75 percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, and a surface area of at least 30 sq. m/cc.
 7. An unbacked polymeric copy which consists essentially of open-celled microporous polymeric film images characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above 20 percent, a pore size of 100 to 12,000 Angstroms, a nitrogen flux of about 5-400 and an elastic recovery at 50 percent extension of greater than 40 percent; upon a non-porous, transparent background of the same polymeric material, said film being further defined by having a plurality of elongated non-porous, interconnecting surface regions which have their axis of elongation substantially parallel; substantially alternating with and defined by these non-porous surface regions a plurality of elongated porous surface regions which contain a plurality of parallel fibrils which are connected at each of their ends to the non-porous regions and are substantially perpendicular to them; and between the fibrils pores or open cells which are substantially interconnected through torturous paths which extend from one surface region to another surface region.
 8. A polymeric copy according to claim 7 wherein the open-celled microporous film image comprises a film of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, a bulk density of 50 to 75 percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, and A surface area of at least 30 sq. m/cc. 